The desire for propulsion systems which have little or no draft is a universal need. Water jets and surface drives are two embodiments of marine propulsion systems which have shallow draft as one of their defining characteristics.
Jets are essentially water pumps that have been fitted into water craft. Water is brought into the an inlet which is generally composed of an upswept bend which lifts the water and another bend changes its direction and brings it to the face of the impeller. The impeller increases the pressure in the water column which is needed to expel the water through a nozzle. This event produces thrust to propel the craft.
However, water jets have always been relatively inefficient when compared to open propellers and efforts to improve their efficiency over the last 30 years have met with limited success. Top speed is still well below the normal attainment of simple open propellers.
Air ingestion is also a problem when rough seas are encountered. Jets generate pressure by slightly compressing the water they intake. This makes them sensitive to relatively small amounts of air in the incoming stream. Pratt & Whitney testing showed that when air volumes in the inlet flow are just a few percent of the total inlet flow, pump efficiency drops off dramatically. In normal use in heavy seas air ingestion is difficult if not impossible to eliminate from the inlet and so jets tend to unload when air is ingested and lose thrust. To combat this problem the inlets so jets tend to be kept as short as possible but often this length deficiency hurts performance due to excessive separation or turbulence in the inlet stream capture volume.
Surface drives are another way to reduce draft by bringing the propeller up vertically and operating it with only partial submergence. While submergence varies, 50% of the propeller diameter is a good average. In fixed surface drives steering is accomplished through the use of a rudder behind the propeller just like an inboard. In moveable shaft systems the propeller can be steered and trimmed for improved high speed operation.
Unlike jets, surface drives have excellent efficiency. Efficiencies as high as 85% have been documented for inclined shaft, 50% submerged super-cavitating propellers. Inclined super-cavitating propellers are the most efficient form of marine propulsion known.
What is lacking is a product which takes the speed advantages and efficiency of surface drives and joins them with the shallow draft and protection from underwater impact damage afforded by water jets.
A significant amount of prior art exists which relates to raising the propeller of a marine vessel vertically up into the hull for the purpose of avoiding or minimizing impact with an underwater obstruction.
The surface drive, invented by Hickman in his U.S. Pat. No. 1,044,176 dated Nov. 12, 1912 sought to raise the propeller of a conventional inboard up vertically to as to reduce appendage drag and improve top speed. Many ideas followed with different approaches to utilizing the basic advantages of surface drives. Many of these concepts extend the propeller out the stem of the boat in either a fixed shaft mechanism or a moveable shaft mechanism but some designers sought to bring the surfacing propeller forward of the transom and into the hull into what is generally termed the tunnel.
Once this evolution became accepted designers sought ways to further reduce the amount of propeller which extends below the bottom of the hull without paying a performance penalty. Unfortunately, a performance penalty is always paid if the propeller requires incoming water to be lifted up vertically in order to operate.
Generally when a surfacing or super-cavitating propeller is brought up into a tunnel, the tunnel is at least partially filled with air. When air is introduced into the hull channel ahead of the propeller there are often methods for controlling the source, point of introduction and amount of air.
Examples of tunnel systems which are air in a tunnel with a super-cavitating propeller can be found in prior art such as U.S. Pat. Nos. 1,534,725; 3,604,385; 3,745,963; 3,793,978; 4,015,556; 4,027,613; 4,371,350; 4,406,635; 4,655,157; 4,689,026; 4,941,423; 4,977,845,
U.S. Pat. No. 1,534,725 by Macmillan shows a set of counter-rotating propellers (van wheels) which operate in a cavity which is filled with pressurized air to maintain the level of water in the cavity desired. Macmillan does not raise the propeller blades above the propeller which is approximately 40-50% submerged. Macmillan also does not propose a hull channel to feed the propeller water.
U.S. Pat. No. 3,604,385, High Speed Water Craft by Mickleover concerns a high speed water craft which utilizes a tunnel with a propeller disposed longitudinally inside the tunnel. Mickleover states that the forward portion of the tunnel rises clear of the water level when the craft is on plane to admit air into the tunnel and relieve the suction from underneath the hull. While there is a secondary source of air for relieving the suction (the engines exhaust) it is clearly understood that the primary source of air to ventilate the tunnel is coming directly into the front of the tunnel cavity when the forward portion of the tunnel rises clear of the natural water surface.
The current invention seeks just the opposite of Mickleover by preventing air from entering the hull channel from the front of the boat. Unlike Mickleover it is the intent of this patent to have the abrupt wall located in the tunnel to be positioned below the surface of the water under normal operation and to achieve this we position the step in the tunnel further aft, closer to the propeller and lower, vertically so that under normal conditions the step in the hull channel is under water due to the normal submergence of the hull when on plane. This is important to understand. The Mickleover approach fails to control the submergence of the propeller in rough water conditions. Waves traveling down the forward tunnel section will cause uneven loading of the propeller.
U.S. Pat. No. 3,745,963 by Fischer has propellers in cavities in the hull with the tunnel aft of the top half of the propellers and the propeller shafts extending below the running surface of the hull.
U.S. Pat. No. 3,793,980, by Sherman, details a tunnel mounted super-cavitating propeller which is positioned with approximately half of the propellers effective diameter below the running surface of the hull. At slow speed the tunnel is filled with water which is pulled up into the tunnel through a series of slots located in front of the propeller. U.S. Pat. No. 4,027,613 by Wollard shows a stepped hull with a surfacing propeller aft of the step. No hull channel exists for feeding the surfacing propeller.
U.S. Pat. No. 4,057,027 by Foster shows a super-cavitating propeller in a pocket near the stem with a short water supply duct.
U.S. Pat. No. 4,406,635 by Wuhrer seeks to improve the construction of the mechanism which is used to move the flow control plate in the Kruppa patent.
U.S. Pat. No. 4,655,157 by Sapp shows a stepped hull which pivots at the trailing edge of the running surface located some distance fore of midship. There is no hull channel to feed water to the super-cavitating propeller.
U.S. Pat. No. 4,689,026 by Small shows a super-cavitating propeller in a tunnel with air. There is no hull channel feeding the propeller water. The position of the propeller is approximately 50% submerged.
U.S. Pat. No. 4,977,845 by Rundquist builds on the concept by Kruppa and Wurher to control propeller submergence and boat handling with movable flaps.